1. Field of the Invention
The present invention relates to a step-zoom lens which is specially designed to change its variable focal length stepwise.
2. Description of the Related Art
The assignee of the present invention has proposed a step-zoom lens system having a distance-varying lens group in which the distance between two lens groups varies at an intermediate focal length. Specifically, this zoom lens system includes a plurality of movable lens groups which are moved to vary the focal length of the zoom lens system, and at least one lens group of the plurality of movable lens groups includes two sub-lens groups serving as a switching lens group. One of the two sub-lens groups is moveable, along the optical axis of the zoom lens system, to be selectively positioned at one of the movement extremities of the moveable sub-lens group with respect to the other sub-lens group. In a short-focal-length side zooming range covering the short focal length extremity over an intermediate focal length, the moveable sub-lens group is arranged to position at one of the movement extremities thereof. In a long-focal-length side zooming range covering the long focal length extremity over the intermediate focal length, the moveable sub-lens group is arranged to position at the other of the movement extremities thereof. The moving path of the switching lens group having the two sub-lens groups, and the moving paths of the other lens groups of the plurality of movable lens groups are discontinued at the intermediate focal length. The zoom lens system is arranged to form an image on a predetermined image plane in accordance with a position of the moveable sub-lens group. This zoom lens system is disclosed in Japanese Unexamined Patent Publication No. 2000-275518.
On the other hand, a step-zoom lens which changes its variable focal length stepwise, without changing throughout a continuous range of focal lengths, for the sake of simplifying the focal-length control and the focus control mechanism of the zoom lens is known in the art. This step-zoom lens is provided with a cam ring having a set of cam grooves. Each of these cam grooves is designed to have a finite number of focal-length steps determined to follow a reference cam diagram for moving the focusing lens group to perform a zooming operation for an image at infinity, and is also designed to perform a focusing operation between an infinite photographing position and a closest photographing position at each of the aforementioned focal-length steps by rotation of the cam ring.